A new flux clean method of using DI water to replace organic solvent

Power QFN use solder paste attach power die to achieve high electrical conductivity and high thermal conductivity, but the flux which come from solder paste can contaminate lead frame surface during reflow process. We also have flux clean process to remove flux residue by organic solvent. However, the flux residue can´t be removed completely by the flux clean organic solvent and it can lead to down bond non-stick and low wire peel strength issue etc. In this paper, we introduce a new flux clean method which is more convenient and less environment pollution. Firstly, covers the whole L/F surface by surfactant and shaped a protected coating before die bond and reflow process. And then, the flux residue will drop on the coating during reflow process. Secondly, use DI water to take place of organic solvent to do the flux clean process. Due to the surfactant can be dissolved with DI water, so the surfactant can be removed by DI water meanwhile the flux residue can be removed too. After comparing two flux clean method, the new flux clean process performance is better than old flux clean process, and no impact for solder void; wire peel strength; delamination between lead frame and compound. Moreover, new flux clean process is lowering cost, less environment and more convenient.